Distance Measuring Equipment (DME) is a pulse ranging system as illustrated in FIG. 18, used to measure the slant range between the airborne interrogator 1804 (typically on an aircraft 1802 or satellite) and the ground transponder 1812 (typically with an antenna 1810 to transmit the pulses). DME is typically categorized as DME/N (Normal) and DME/P (Precision). These two systems use different pulse shapes that support their own operational purposes. The DME/N has been used for traditional rho/theta as well as for en-route and limited terminal area navigation (RNA V). On the other hand, the DME/P use is limited to Micro Landing System (MLS) approach and landing applications. Current DME specifications do not require a particular pulse shape, but specify the allowable ranges of pulse shape parameters such as rise time, width, and fall time.
The common pulse shape used for DME/N is a Gaussian pulse. The Gaussian pulse has a rise time of 2.5 μs and a narrow spectral density that can be transmitted up to 1,000 Watts. In turn, the typical DME/P pulse shape is the Cos/Cos2 pulse. The Cos/Cos2 pulse has a much faster rise time than the Gaussian pulse, which provides the much higher range accuracy. However, this higher accuracy comes at the expense of the increased spectral density, which limits its transmission power to 100 Watts to prevent interference on adjacent channels. Thus, the coverage of the DME/P ground station is substantially smaller than that of a DME/N ground station.
Therefore, there is a need in the art for improvements in DME systems.